This is a continuation application, centering on the role of HIV capsid protein (CA) in virion structure and post-assembly events, and how the host protein cyclophilin A (CyP A) facilitates this role. The specific aims of the projects are: AIM 1: to identify the basis of Gag (precursor) and CA recognition by cellular chaperons such as CyP A and to determine the role of CyP A in maturation of the Gag precursor. AIM 2: to identify the CyP A-dependent CA proteins in viral particles and to determine their function, through reconstitution experiments. AIM 3: to define, using X-ray structure and genetic analysis, the structural changes that result from CyP A-induced alterations of the mature CA protein. AIM 4: to distinguish regions in the Gag and CA proteins involved in different CyP A interactions, using mutants defective in folding and assays distinguishing the chaperon versus isomerization activities of CyP A.